Queries a Tree from MMBase. A Tree is presented as a List of MultiLevel results (ClusterNodes),
combined with a smart iterator which iterates through the elements of these lists as if it was one
list ordered as a Tree.

grow

Grows branches of the Tree, which means that one new query will be created which is one
relationStep longer than the longest one until now.
This new relationStep is returned, which can be used to create new constraints.

Returns:

null if no relationstep is added because that would not increase the number of results.

setLeafConstraint

Sets a 'leaf constraint' on the last 'growed' step. A leaf constraint is a constraint which is only
used on leafs, so if the tree is grown further, the leaf constraint will not be passed to the branches.